Re: Changing/creating apple id on iPhone of family member
Here is a nice step by step process from the Apple Support page for you. I also linked to a family sharing document which you might find useful.
Create an Apple ID on an iPhone, iPad, or iPod touch
From the Home Screen, tap Settings > iTunes & App Stores.
If an Apple ID is already signed in, tap the Apple ID name, and then tap Sign Out.
Tap Create New Apple ID.
Choose the appropriate store by tapping Store, and select the country in which you are located.
Tap Done to confirm your selection, and then tap Next.
To proceed, agree to the Terms and Conditions and Apple Privacy Policy. You can opt to have the Terms and Conditions and Privacy Policy emailed to you for easier reading by tapping Send by Email. To do this, enter a valid email address, and then tap Send.
Tap the Agree button on the bottom-right of the page.
Tap Agree again when the confirmation appears.
After you've agreed to the terms and conditions, you'll need to enter an email address and password, and choose three security questions and provide answers to them. The email address you provide will be your new Apple ID. Enter your email address, and then tap Next.
You can also enter an optional rescue email address. We recommend providing a rescue email address.
If you're trying to create an account using an existing Apple ID, like your iCloud email address, you'll receive a prompt telling you that your email address is already an Apple ID. Simply tap Continue, and then tap Use Existing Account on the following screen.
Next, you'll be asked to enter your billing information. To choose a different Credit Card type, tap one of the available options, and then tap Done. You will not be charged until you make a purchase.
After you've filled out all of the fields for your billing information, tap Next.
You'll then see a screen that says Verify Your Account.
Tap Done, and check your email for a verification email from Apple. If you need to set up email on your iPhone, iPad, or iPod touch, learn how to add an email account to your iOS device.
Open the email, and tap the Verify Now link enclosed in the email to activate your account.
You'll be taken to a secure page to enter your Apple ID and password.
After you've entered your email address and password, tap Verify Address.
To modify or change your account information, such as Apple ID, payment information, or billing address, tap your account name located at the bottom of the page once you're signed in to the iTunes Store. When prompted, tap View Account.
You might be asked to enter your account password again for verification.
To sign out of your iTunes Store account, simply tap your account name at the bottom of the page in the App Store or iTunes Store.
https://support.apple.com/en-us/HT204316
This article will help you set up Family Sharing which you might find useful for you and your son.
https://support.apple.com/en-us/HT201088
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